Allen ADC Amp Issues

[samibe]

My ADC amp has started failing today. I lost channel two today while practicing. I pulled out my guitar amp to check the audio and found that the signal makes it to the amp just fine.

So far, I have tried exercising the amp pot and checking the plugs. That seemed to help the first time the channel dropped out but but not for any of the other times. The channel comes back if the organ is turned off for a while. Then it works for twenty minutes before dropping out again.

I read a couple of the ADC amp troubleshooting threads.
ADC amplifier issues
Allen ADC amp module repair

I'm wondering if there might be a component that's starting to fail (like the resistor in the first thread) but I haven't seen or smelled any blue smoke. Or maybe there's a cold solder somewhere in it.

Any ideas on what I should look out for when I get around to pulling the amp out and looking at it would be appreciated.

[tucsondave]

The failure I've seen most often is the mute relay.
Unplug the amp then with the organ on and the ADC volume controls at max, compare the resistance from each input to ground after the mute signal is on.
A stuck relay would show about 1k, a good relay about 100k.

td

[jbird604]

I was thinking the same thing as TD -- the mute relay. It's possible that the "delay" voltage coming from the organ that trips these relays is not as high as it should be, or that it is going down a bit after the organ warms up due to some component failure in the circuit. This lowered voltage allows the relays to collapse, thus muting the audio.

To check for this, put your voltmeter on the + and - "mute" terminals on the amplifier's terminal strip. Then turn on the organ. You should see nearly zero volts at turn-on, then about 5 seconds later it should jump up to between 10 and 11 volts. It should stay at that level as long as the organ is on, but if there is problem with the delay voltage circuit the voltage may drop after a while.

Every relay has a slightly different pull-in voltage, so that one amp's relay might just have a slightly higher voltage need than the others in your system, so it's the one that is currently failing. In time other channels may go out as well, if this is the problem.

The fix will be to replace the delay circuit inside the AC distribution box in the organ floor. Or build one of my home-made replacements that is fail-proof.

Alternatively, if the voltage is only dropping to about 9 volts and the rest of the relays are fine with it, you might just replace the one mute relay in the one amp with a new one, and it will probably pull in with the reduced voltage.